The present invention relates to a manufacturing method for a color picture tube which has a long life and is highly reliable because gases evolved from each part, even in a large tube during operation are decreased in quantity and the electron emissivity is left unchanged over a long period.
The color picture tube is usually equipped with a phosphor screen on the internal panel surface, internal conductive coating on the internal funnel surface, a shadow mask, an inner shield, an electron shield, and electron gun electrodes in the glass tube. During an exhaustion step or on operation, unnecessary and harmful gases are evolved from the surface of each part mentioned above, resulting in a reduction in the electron emission characteristics of the cathode and in the life span. The phosphor screen evolves structurally a large quantity of gases; consequently, the panel is subjected to the so-called panel baking for out-gassing before the panel and the funnel are subject to frit-sealing. Iron parts including the shadow mask are coated with a corrosion preventing black film comprising of ferrosoferric oxide on each surface. The inner shield is made of an iron plate which is formed with an aluminum film containing silicon on its surface to prevent scattering of impinged electrons and gas evolution, and which is heated in a vacuum to form a black film. This is indicated in Japanese Patent Laid-Open 126524/1987. The electron shield is usually made of aluminum and subjected to corrosion preventing treatment with an oxide film on its surface. The purpose of the corrosion preventing treatment for the surface of each metallic part mentioned above is to prevent each bare surface in the atmosphere from chemical changes such as excessive oxidation which may cause gas evolution during the exhaustion step or on operation.
It goes without saying that gas evolution on operation is undesirable. In addition, at the exhaustion step to be executed after the sealing step for the stem to support each electron gun, gases evolved from the parts in the tube are not always discharged from the tube but a part of them is re-adsorbed in the tube; consequently, excessive gas evolution at the exhaustion step is undesirable. A part of gases evolved from the shadow mask, for example, may be re-adsorbed into the inner shield, the internal graphite coating or the like before the gases are discharged out of the tube and may remain in the tube finally.
To reduce the gas evolution at the exhaustion step and on operation, various countermeasures including complicated ones depending on the treatment process have been taken for the parts in each color picture tube. However, there is recently an increasing demand for large tubes, and the reduction of gas evolution is a more important problem. This is because the gas evolution amount increases as the surface area of each part increases. The conventional countermeasure for such a status is to increase the getter yield. Increasing the getter yield in correspondence with the large size of tubes causes however an undesirable increase in stray electron emission or in the total weight of the getter including the container.